The average monthly electricity use of the 8 flats in a wing is 240 units. One flat, which uses 380 units a month, is disconnected. What is the average monthly use of the 7 flats that remain?

The eight flats use $8\times 240=1920$ units; removing 380 leaves 1540 units for 7 flats, so the new average is 1540/7 = 220 units. The quick route: the departing flat sits 380 - 240 = 140 units above the average, and that surplus is now shared out among the 7 that remain, so the average falls by 140/7 = 20. 192.5 divides the new total by the OLD count of 8. 222.5 spreads the 140 surplus over 8 instead of 7. 260 adds the 20 instead of subtracting it, though dropping the heaviest user must pull the average down.